Wheel alignment is a process of adjusting the angles of wheels on a vehicle so that they are generally perpendicular to the ground and parallel to one another. The purpose of such adjustments is to attain maximum tire life, as well as to ensure that the vehicle tracks straight when driving along a straight and level road.
In order to adjust the wheel angles to achieve proper wheel alignment, the actual wheel angles must first be measured, such that the requisite adjustments may then be calculated. Two particular angles that are utilized in wheel alignment methods are commonly referred to as “camber” and “toe.” Camber, which is typically measured in degrees, is the angle of the wheel's deviation from a vertical plane. Therefore, camber is angle of the wheel that is seen when viewed from the front of the vehicle. If the top of the wheel is leaning out from the center of the car, the camber is positive; if it is leaning in toward to center of the car, then the camber is negative. Toe may be defined as the distance difference between the front of a pair of tires and the back of the pair of tires, in which case the measurement is linear, in units of either inches or millimeters. Alternately, toe may be defined as the angle between a pair of tires (total toe), or as the angle of a tire relative to the vehicle's longitudinal direction (individual toe), in which case the measurement is angular and the units are in degrees. Wheel alignment systems often involve mounting measurement instruments to the wheels and using an angular measurement method. Toe is usually set close to zero degrees, meaning that the wheels are substantially parallel to one another. “Toe-in” means that the fronts of the tires are closer to each other than the rears; “toe-out” is the opposite situation.
Some types of wheel alignment procedures involve placing instrumentation on each of a vehicle's four wheels. The instrumentation may be assembled together in a “head” that is clamped to each of the wheels. A head may include a transmitting device such as an LED emitter, and a receiving device such as a photosensor. During an alignment procedure, the receiving device of each head “looks” at the transmitted light from two heads of the two adjacent wheels. The optical “box” that is formed around the four wheels by the transmitting devices may thus be sensed by the receiving devices as the wheels are rotated, and the various wheel angles may thereby be calculated.
Unfortunately, such procedures involve certain inherent measurement inaccuracies and can therefore result in alignment errors. For example, if a head is clamped to a wheel incorrectly, inaccurate measurements may result. Also, wobble in a rotating wheel may cause inaccuracies in the direction of either the transmitter or the receiver in any of the heads. Wobble, hereinafter referred to as “runout,” is a condition in which the wheel experiences a varying camber and toe as it rotates about its axis. Because runout may affect the camber and toe measurements at any particular point in a wheel's rotation during an alignment procedure, it may result in an inaccurate alignment.
Therefore, wheel alignment methods of the past have attempted to compensate for runout in order to produce more representative angle measurements and achieve a more accurate alignment. However, these “runout compensation” methods may cause a disruption in the car's balance, thereby compromising the alignment. For example, some known runout compensation methods involve lifting the wheels of a vehicle above the alignment surface, and rotating the free wheels while measuring runout (i.e., an elevated runout compensation). After the runout is determined, the wheels are returned to the alignment surface, and the remaining portions of the wheel alignment procedure are performed, using the determined runout for any necessary compensation. However, when a vehicle is lifted off of an alignment surface and then set back down again, it likely does not settle back to its natural position. Rather, it may be some amount of time before the vehicle does settle back to its natural position and retain its original balance. Thus, although potential runout may be compensated for by lifting the wheels off of the alignment surface to spin them and obtain measurements of their runout, such methods in turn create the potential for performing an alignment on a car that is now not settled to its natural stance, which itself may result in an improper alignment.
What is needed is a wheel alignment method that allows for runout compensation without affecting a vehicle's stance. Specifically, what is needed is a method for performing accurate wheel alignments in which runout compensation is achieved while the wheels remain in constant contact with an alignment surface, thereby retaining the vehicle's natural stance.